Almost all modern day automobiles include a powertrain that is comprised of an engine and/or motor in power flow communication with a final drive (e.g., differential and wheels) via a multi-speed power transmission. The multi-speed power transmission requires a supply of pressurized fluid to properly operate. The pressurized fluid may be used for such functions as cooling, lubrication, and torque converter operation. It is well known that the lubricating and cooling capabilities of transmission oil systems greatly impact the reliability and durability of the transmission. Additionally, multi-speed transmissions require the hydraulic system to provide controlled engagement and disengagement, on a desired schedule, of the various torque transmitting mechanisms that operate to establish the speed ratios within the internal gear arrangement.
One premise behind hybrid-type vehicles is that alternative power is available to propel the vehicle, minimizing reliance on the engine for power, thereby increasing fuel economy. Since hybrid-type vehicles can derive their power from sources other than the engine, engines in hybrid-type vehicles typically operate at lower speeds more often than their traditional counterpart, and can be turned off while the vehicle is propelled by the alternative power source(s). To this regard, when the engine in a hybrid-type vehicle is not operating (i.e., in a power-off state), hydraulic pumps which derive their power solely from the engine may become inoperable. As such, many hybrid powertrains include an electrically-driven secondary or auxiliary pump that runs independent of the engine—e.g., powered by the vehicle drive lines or the battery, to provide hydraulic pressure during periods when the engine is shutdown.
For fuel economy reasons, vehicle system voltage is typically set as low as possible, an undesirable situation for some transmission auxiliary pumps. Permanently setting the vehicle system to a higher voltage—for example, not tying the voltage increase directly to a hybrid engine-off mode transition, is also undesirable for reasons of fuel economy, because the higher voltage would increase overall vehicle power consumption. However, a transmission auxiliary pump which fails during transition to hybrid “engine-off driving mode” can potentially result in slip in the transmission launching clutch, and may trigger a “limp-home code” in production.
Packing space in and around the powertrain in hybrid-type vehicles is often scarce, often restricting use of a larger auxiliary pump motor. In addition to architectural limitations, installing a larger pump motor is not always possible due to mass, cost, and fuel economy constraints. As such, the motor of a transmission auxiliary pump may be so small that it may not be able to start reliably under certain conditions.